A thin film solar cell technique is an advanced solar cell technique that is compared to a technique of crystalline silicon (Si) solar cell which currently has the largest market share, wherein the thin film solar cell has a higher efficiency than the crystalline Si solar cell and may be manufactured at a lower cost.
Various types of thin film solar cells have been developed and a typical example thereof may be a Cu(In,Ga)Se2 (CIGS) solar cell.
The CIGS solar cell denotes a cell that is composed of general glass substrate-back electrode-light-absorbing layer-buffer layer-front transparent electrode, in which the light-absorbing layer absorbing sunlight is formed of CIGS or CuIn(S,Se)2 (CIS). Since the CIGS is more widely used among the CIGS or the CIS, the CIGS solar cell will be described hereinafter.
Since CIGS, as a group I-III-VI chalcopyrite-based compound semiconductor, has a direct transition type energy bandgap and a light absorption coefficient of about 1×105 cm−1 which is one of the highest among semiconductors, the CIGS is a material capable of manufacturing a high-efficiency solar cell even with a 1 μm to 2 μm thick thin film.
Since the CIGS solar cell has electro-optically excellent long-term stability even at outdoors and excellent resistance to radiation, the CIGS solar cell is suitable for a spacecraft solar cell.
In general, glass is used as a substrate of the CIGS solar cell, but the CIGS solar cell may be manufactured in the form of a flexible solar cell by being deposited on a polymer (e.g., polyimide) or a metal thin film (e.g., stainless steel, titanium (Ti)) substrate in addition to the glass substrate. In particular, the CIGS solar cell, as a low-cost, high-efficiency thin film solar cell, has been known as a solar cell having a very high commercialization potential which may replace a crystalline silicon solar cell, as the highest energy conversion efficiency of 19.5% among thin film solar cells has been recently realized.
CIGS may be used by replacing cations, such as copper (Cu), Indium (In), and gallium (Ga), and an anion, such as selenium (Se), respectively with other metal ions or anions, and these materials may be collectively referred to as a CIGS-based compound semiconductor. A representative compound of the CIGS is Cu(In,Ga)Se2, and the CIGS-based compound semiconductor is a material in which its energy bandgap as well as crystal lattice constant may be adjusted by changing types and compositions of constituting cations (e.g., Cu, silver (Ag), In, Ga, aluminum (Al), zinc (Zn), germanium (Ge), tin (Sn), etc.) and anions (e.g., Se and sulfur (S)).
Thus, a light-absorbing layer formed of a similar compound semiconductor material including a CIGS material may also be used. The light-absorbing layer may include a compound which includes M1, M2, X, and a combination thereof (where M1 is Cu, Ag, or a combination thereof, M2 is In, Ga, Al, Zn, Ge, Sn, or a combination thereof, and X is Se, S, or a combination thereof). Recently, for example, a material, such as Cu2ZnSnS4 (CZTS) or Cu2SnxGeyS3 (CTGS), may also be used as a low-cost compound semiconductor material (where x and y are arbitrary prime numbers).
Even in a typical thin film solar cell, a technique for further increasing efficiency through a combination with a piezoelectric device has been developed.
For example, the following Patent Document 1 by Wang et al. suggests a method of improving efficiency of a hybrid solar nanogenerator, in which a charge generated by mechanical vibration is collected by installing a piezoelectric nanogenerator using a ZnO nanowire on an electrode of a dye-sensitized solar cell in series or in parallel to contribute to power generation with photocurrent. However, since a technique disclosed in the following Patent Document 1 additionally requires energy and equipment to generate the mechanical vibration, economic efficiency may be reduced.
Also, in the following Patent Document 2, a solar cell technique capable of improving light conversion efficiency by an electric-field enhancement effect is disclosed in which the technique is for improving photoelectric conversion efficiency of a solar cell by effectively transferring electrons and holes, which are generated from a photoactive layer due to the light, by installing a field emission layer, which includes a nanostructure in the form of a nanorod, a nanowire, or a nanotube having a field emission effect, on an electrode of the thin film solar cell. However, as a result of being applied to various actual thin film solar cells, an efficiency improvement effect may be insignificant and processing costs for fabricating the nanostructure may be increased. Thus, similar to the technique disclosed in Patent Document 1, economic efficiency may be reduced.